This invention relates to protection apparatus for electrical equipment and particularly to series capacitors protected by solid state varistors which in turn are to be protected against energy inputs beyond their capability.
Resistors that are highly non-linear, such as metal oxide varistors usually comprising zinc oxide with smaller amounts of other metal oxide constituents, are used to limit the magnitude of voltage across electrical devices that are sometimes subjected to high voltages due to system faults. The varistors, in a permanently connected circuit branch across the protected equipment without any spark gaps in that branch, provide the ability to clip the voltage across the protected equipment to a safe level while keeping the protected equipment connected and in service for its intended function. This is the case with series capacitors used in long AC transmission lines carrying high voltages where the capacitors are directly in series with the transmission line to compensate for inductive reactance and to provide other improvements in electrical characteristics.
Series capacitors are subject to a range of possible overvoltage conditions. The series capacitors themselves can withstand safely a certain level of overvoltage, for example, up to about twice the nominal voltage, referred to as 2 P.U. (per unit). For overvoltages beyond the capability of the capacitors themselves, they need to be protected. Varistors connected in a circuit branch in parallel with the capacitors provide a means for doing so. The varistors can be designed so that at the design limit of the capacitors, say 2 P.U., they start to conduct and effectively remove the excess voltage. This permits the capacitors to remain on the line and to provide all of their intended functions during all portions of the AC cycle in which the voltage does not exceed 2 P.U. In concept, the amount of varistor material used in the assembly can be selected to enable the handling of any contemplated fault condition. Practical limitations often prevent the application of this concept. For example, high fault current conditions may be caused by faults located close to the series capacitor, where the line breakers will isolate the faulted line segment. At the present state of the art, the provision of varistors to handle such large currents would require inordinantly large amounts of varistor material that would be exceedingly expensive. That is because varistors have limits on their energy handling capability above which thermal destruction may result.
To permit a more economical use of varistors in protecting series capacitors, the system can include a bypass device such as a spark gap for their protection at a safe level, such as say, 3 PU of current. Hence, an overcurrent of that amount would result in firing of the bypass spark gap to protect the equipment, but at the same time, effectively remove the capacitors from service for a time. For example, in application Ser. No. 126,094, filed Feb. 29, 1980 by C. A. Peterson, assigned to the present assignee and now abandoned, is described further background on the subject and a means for firing a bypass spark gap under certain conditions. In that patent application, the current through the varistors is sensed to develop a voltage that is then applied through a step-up potential transformer to a trigger spark gap so as to respond to a predetermined voltage level and result in the firing of the principal spark gap bypass. Other systems devised for firing bypass spark gaps around varistors protecting series capacitors are contained in Hamann U.S. Pat. Nos. 4,174,529 and 4,259,704, issued Nov. 13, 1979 and Mar. 31, 1981, respectively. In general, such systems operate in a peak current dependent manner; that is, they can initiate the bypass spark gaps firing upon a given magnitude of current passing through the varistors. When it comes to the time dependent nature of thermal buildup, due to energy input in the varistors, they are relatively less effective. U.S. Pat. No. 4,174,529 contains an approach in which a current sensing device coupled with a combined thermal analog and a low voltage pulse generator circuit generates a low voltage pulse for initiating the operation of a high voltage pulse generator to trigger an air gap device bypassing the varistors. The thermal analog circuit of the patent consists of a resistor-capacitor charging circuit in which the discharge time constant of the RC circuit is relied on for approximating thermal recovery of the varistor. This does so only in a crude way. RC circuits are suitable for thermal analogs for only short time periods because it would be difficult to devise a capacitor whose discharge rate reliably approximated the thermal recovery of varistors over an extended time such as about 30 minutes or more.
The present invention obviates the difficulties and deficiencies of the prior art apparatus by utilizing digital integration apparatus for initiating a firing signal for the bypass spark gap and, preferably, utilizing the initial firing signal in an improved gap firing system that reliably ensures that the initial firing signal, though of relatively low voltage, will rapidly result in the firing of the spark gap. The digital integration apparatus, in brief, and its method of operation, involve the development of a pulse train, for example, a series of digital "one" signals, from the monitored varistor current in which each pulse occurs when a voltage directly related to the sensed current is integrated over time to produce a small predetermined increment, Vdt. The digital pulse train is producible at such a high rate, such as in excess of 10,000 pulses per second, that it offers wide flexibility in its use as a time dependent measure of energy in the varistors. Firing signals can be generated upon the occurrence of any of a multiplicity of different conditions. Some of these conditions may, for example, relate to relatively short time events in which a number of pulses within periods down to the millisecond level can be reliably counted and if they reach a threshold can promptly initiate the firing signal. The same pulse train can be applied to each of a number of counter sets that are timed for different conditions. In addition, thermal analoging is unnecessary by digitizing the energy input so that no analog devices such as long time constant capacitors are required. Yet, the ability to simulate the exponential cooling rate of the varistors over substantial periods of time such as about 30 minutes or more is provided.
The pulses of the pulse train for these various functions can be counted by any number of counters which are in turn individually reset according to prescribed time intervals. In addition, for thermal digitizing, a counter of the type shown in digital signal processing as an up/down counter is provided with the pulse train, to increase the count in the counter, while also being provided with inputs from a time regulated signal source, such as a crystal oscillator and timing module in conjunction with a binary rate multiplier or another device providing the same function, to decrease the count in a manner accurately related to the cooling rate of the varistors so that the stored count at any instant in the up/down counter represents the energy storage of the varistors.
What is achieved by these techniques is greatly increased versatility and reliability of the firing system so that as a consequence, for a given quantity of varistors, say sufficient to handle 2 PU without risk of harm, the initiation of a firing pulse for the bypass spark gap can be safely delayed, in order to keep the capacitors in service, until conditions truly require their removal while at the same time permitting a recognition of a build-up of energy under high fault conditions where there is a rapid rate of rise of current so that even without approaching the limit of the varistor's capability, the gap will fire. This latter function is of significance so that upon firing of the gap, the system may be restored to operation as rapidly as possible. To do so, one would wish that the varistors not have been subjected to as high energy input as they may be capable of sustaining because such conditions result in heating which would require a further time for dissipation before permitting the operation of the system. Consequently, the system provides reliable operation under a variety of short and long time conditions.
Additional features of the improved system have to do with the means and manner in which the spark gap is fired after the initial firing signal is produced by the digital system. The presently preferred arrangement includes a solid state switching device such as a thyristor which normally blocks the conduction in a primary winding of a transformer. In the secondary winding of that transformer is a trigger spark gap which can be highly precise to rapidly fire upon the initiation of conduction in the primary. An energy storage system receives power from a power supply that is in operation at all times the transmission line is energized and comprises an energy storage capacitor that is in a circuit branch including the trigger spark gap and part of the main gap firing system. The latter includes a resistor connected across two electrodes of a three electrode spark gap. The voltage built up across this resistor upon the firing of the trigger gap and the release of energy of the energy storage capacitor is sufficient to break down that spark gap and result in a cascading effect, where multiple spark gaps are used, particularly in conjunction with a cascading capacitor system for total conduction of the bypass parallel to the varistors. The system is normally provided with additional components as will be described hereinafter, such as a bypass breaker in another parallel branch which may be manually operated or may be responsive to current conduction through the spark gap bypass to close and provide a further conduction path.